The graphical modeler allows to create complex models using a simple and easy-to-use interface. When working with a GIS, most analysis operations are not isolated, but part of a chain of operations instead. Using the graphical modeler, that chain of processes can be wrapped into a single process, so it is easier and more convenient to execute than a single process later on a different set on inputs. No matter how many steps and different algorithms it involves, a model is executed as a single algorithm, thus saving time and effort, specially for larger models.
The modeler has a working canvas where the structure of the model and the workflow it represents are shown. On the left part of the window, a panel with two tabs can be used to add new elements to the model.
Creating a model is a two-step process.
The first step to create a model is to define the inputs it needs. The following elements are found in the Inputs tabs on the left side of the modeler window:
Double-clicking on any of them, a dialog is shown to define its caracteristics. Depending on the parameter itself, the dialog will contain just one basic element (the description, which is what the user will see when executing the model) or more of them. For instance, when adding a numerical value, as can be seen in the next figure, apart from the description of the parameter is needed to set a default value, the type of numerical value and a range of valid values.
For each added input, a new element is added to the modeler canvas.
Once the inputs have been defined, it is time to define the algorithms to apply on them. Algorithms can be found in the Processes tab, grouped much in the same way as they are in the toolbox.
To add a process, double-click on its name. An execution dialog will appear, with a content similar to the one found execution panel that SEXTANTE shows when executing the algorithm from the toolbox.
Some differences exist, however, the main one being the absence of a raster ouput tab, even if the selected algorithm generates raster layers as output.
Instead of the textbox that was used to set the filepath for output layers and tables, a checkbox and a text box are found. If the layer generated by the algorithm is just a temporary result that will be used as the input of another algorithm and should not be kept as a final result, the check box should be left unchecked. Checking it means that the result is a final one, and you have to supply also a valid description for the output, which will be the one the user will see when executing the model.
Selecting the value of each parameter is also a bit different, since there are importante differences between the context of the modeler and the toolbox one. Let's see how to introduce the values for each type of parameter.
Once all the parameter have been assigned valid values, click on OK and the algorithm will be added to the canvas. It will be linked to all the other elements in the canvas, whether algorithms or inputs, which provide objects that are used as inputs for that algorithm.
Once the model has been designed, it can be executed clicling on the Execute button. The execution window will have a parameters tab automatically created based on the requirements of the model (the inputs added to it), just like it happens when a simple algorithm is executed. If any of the algorithms of the model generates raster layers, the Raster output tab will be added to the window.
Elements can be dragged to a different position within the canvas, to change the way the module structure is displayed and make it more clear and intuitive. Links between elements are update automatically.
To change the parameters of any of the algorithms of a model, double-click on it to acces its parameters window.
To delete an element, right-click on it and select Delete. Only those elements that do not have any other one depending on them can be deleted. If you try to delete an element that cannot be deleted, SEXTANTE will show the following warning message.
Models can be saved to be executed or edited at a later time. Use the Save button to save the current model and the Open model to open any model previously saved. Model are saved in an XML file with the .model extension.
Models saved on the models folder will appear in the toolbox in a group that you can set using the boxes in the top of the modeler window. Type in the name of the model and then select a group from the drop-down list. The list contains all the names of the already existing groups, and also an additional group named ``Models''. If none of this group suits your needs, you can type a new name directly in that box, which is editable.
When the toolbox is invoked, SEXTANTE searches the models folder for files with .model extension and loads the models they contain. Since a model is itself a SEXTANTE algorithm, it can be added to the toolbox just like any other algorithm.
The models folder can be set from the SEXTANTE toolbox, clicking the configuration button and then introducing the path to the folder in the corresponding field. Go to the ``Folders'' tab to find it.
Models loaded from the models folder appear not only in the toolbox, but also in the algorithms tree in the Processes tab of the modeler window. That means that you can incorporate a model as a part of a bigger model, just as you add any other algorithm. however, models are shown with a different icon, to make it easy to recognize them.
By default, the models folder is the same one as the folder where SEXTANTE help files are located. This folder contains a small set of example models, that you can use to better understand how the modeler works. Open them and study how they are constructed. You can also check their associated help files. As it has been said, models are themselves SEXTANTE algorithms, so they can have their own help files, and these can be edited as we have already seen in the previous chapter.